1. Field of the Invention
The present invention relates to a DC-DC converter that feeds a load circuit with a DC voltage at a predetermined magnitude, and more particularly to a DC-DC converter that has an overcurrent protection function.
2. Description of the Related Art
Heretofore, in personal computers of notebook type, portable telephones or any other portable electronic equipment, battery voltage has been stepped down to a prescribed voltage by a DC-DC converter such as a step-down type switching power supply circuit, so as to feed and drive an electronic circuit with the prescribed voltage. Overcurrent protection circuits that protect switching elements from overcurrents have been used in such arrangements.
FIG. 5 is a diagram showing an example of a prior-art DC-DC converter that performs overcurrent protection by limiting the peak of an inductor current IL. This type of DC-DC converter is so configured that a first switching semiconductor element (hereinbelow, termed the “switching element”) Mp, which consists of afield-effect transistor (FET) of P-channel type whose source terminal is connected to an input voltage Vin from a battery, and a second switching element Mn, which consists of an FET of N-channel type whose source terminal is grounded, have their drain terminals connected to each other, and that the gate terminals of the respective switching elements Mp and Mn are fed with control signals through driver circuits 2 and 3 from a switching control circuit 1. One end of each of a charging/smoothing output capacitor Cout and a load circuit 4 is connected to the connection point between the switching elements Mp and Mn, through an inductor L, and the other ends of the output capacitor Cout and the load circuit 4 are respectively grounded.
The inductor L and the output capacitor Cout constitute a filter circuit that feeds a smoothed output voltage Vout to the load circuit 4. In order to detect the output voltage Vout to the load circuit 4, this output voltage Vout is fed to the switching control circuit 1 as a negative feedback signal. While monitoring the output voltage Vout, the switching control circuit 1 outputs control signals to the switching elements Mp and Mn through the corresponding driver circuits 2 and 3 and controls these switching elements so as to alternately turn ON/OFF, in order that the output voltage Vout may become a constant value.
An overcurrent detection circuit 5 senses an overcurrent in such a way that a current flowing through the switching element Mp, namely, the inductor current IL flowing from the switching element Mp into the inductor L is detected by, for example, a current transformer, or a sense resistance and an amplifier, and that the detected current is compared with a predetermined reference value. The DC-DC converter is so configured that, in a case where the overcurrent has been sensed by the overcurrent detection circuit 5, the switching control circuit 1 controls the switching element Mp into the OFF state thereof.
In the prior-art DC-DC converter, during the execution of an ordinary switching operation, the switching control circuit 1 changes the pulse widths of the pulse signals for controlling the switching elements Mp and Mn into the ON and OFF states thereof, in accordance with the change of the output voltage Vout, thereby to perform feedback control so that this output voltage Vout may become constant. Thus, even in a case of a heavy load in which a feed current Iout to the load circuit 4 is large, energy which has been accumulated in the inductor L in accordance with the load during the OFF state of the switching element Mp is emitted through the switching element Mn, so that efficient synchronous rectification can be performed.
When the current (=IL) flowing through the switching element Mp on the side of the input voltage Vin of the DC-DC converter exceeds a prescribed value Ipmax, an overcurrent detection signal is outputted from the overcurrent detection circuit 5, and the switching element Mp is held in the OFF control state till the next switching cycle through the switching control circuit 1. In this way, the overcurrent limitation function of limiting the peak current value Ip of the inductor current IL to, at most, the prescribed value Ipmax by the overcurrent detection circuit 5 is realized.
However, a certain delay time is required for switching and turning OFF the switching element Mp after the current exceeding the prescribed value Ipmax has been actually sensed by the overcurrent detection circuit 5. Therefore, even when the overcurrent state has been sensed the moment the switching element Mp has been shifted into the ON control state by the drive signal from the switching control circuit 1, the inductor current IL continues to increase for a time period until the switching element Mp actually falls into the OFF state through the overcurrent detection circuit 5.
FIGS. 6A and 6B show how the inductor currents change in states in which the overcurrent protection function of the prior-art DC-DC converter is operating, in cases where the output voltage Vout is large and where it is small, respectively. In each of the figures, the ordinate axis represents the inductor current IL, while the abscissa axis represents the time.
Here, let “Td” denote the minimum delay time which is required for bringing the switching element Mp into the OFF control state in the case where the inductor current IL has exceeded the prescribed value Ipmax. Since the decrease rate of the inductor current IL is proportional to the output voltage Vout (dIL/dt=Vout/L), a longer time period is required for the decrease of the inductor current IL in the case where the output voltage Vout is low, than in the case where it is high, as shown in FIG. 6B. On the contrary, the rate of increase of the inductor current IL becomes larger as the output voltage Vout becomes lower (dIL/dt=(Vin−Vout)/L). As shown in FIG. 6B, therefore, the inductor current IL continues to rise at a large gradient with the delay time Td, even after this inductor current IL has exceeded the prescribed value Ipmax. Then, within a time period for which the switching element Mp is under the OFF control, the inductor current IL begins to rise in the next switching cycle, in a state where this inductor current IL having increased till then cannot decrease sufficiently.
In this manner, with the overcurrent detection by the overcurrent detection circuit 5 in the prior art, the overcurrent limitation function for the inductor current IL might fail to effectively operate. Moreover, if the delay time Td since the detection of the overcurrent is constant, it will become more difficult for the overcurrent limitation to function normally as a switching frequency in the switching control circuit 1 continues to rise.
In such a switching power supply, accordingly, an overcurrent protection method for a switching power supply circuit has been considered in which an inductor current (IL) is prevented from increasing at a minimum ON-duty time (Tmin) (refer to, for example, JP-A-2004-364488 (especially, paragraphs [0046]-[0080] and FIGS. 1, 2 and 3), which corresponds to U.S. Pat. No. 7,068,023 B2).
JP-A-2004-364488 discloses a technique including an overcurrent detection circuit (221) that detects a current (I-H) that flows from a transistor (2) being a switching element for an inductor, and a current detection circuit (230) which detects a flywheel current (I-L) at the turn-OFF of the switching element, wherein when it has been detected by the overcurrent detection circuit (221) that the value of the current (I-H) (=inductor current) exceeds a certain predetermined value, the switching operation of the switching element is masked and stopped until it is detected by the current detection circuit (230) that the flywheel current becomes, at most, another predetermined value.
The overcurrent protection method of JP-A-2004-364488 has had the problem that, since two detection circuits are required for detecting the current (I-H), which flows from the transistor (2) to the inductor, and the flywheel current (I-L), respectively, the cost of the switching power supply circuit is increased.
In addition, in an overcurrent protection operation stated in JP-A-2004-364488, it is not supposed to establish synchronization between a timing at which the switching element is unmasked from a drive pulse and a timing at which a PWM pulse for the switching element is subsequently fed for the first time. This poses the problem that, since the minimum value of the inductor current disperses depending upon both the timings, the average value of the inductor currents cannot be predicted, so exact switching control becomes difficult. This problem conspicuously arises especially in a case where the switching frequency of the switching element is low.
Further, with an overcurrent protection circuit that is applied to PFM (pulse frequency modulation) control of a fixed ON-time system, in a case where an output voltage has been reduced by overcurrent limitation, a control circuit performing a constant-voltage control continues to raise a switching frequency with the intention of increasing the output voltage. This poses the problem that the overcurrent limitation function based on the prior-art system does not operate effectively.
Such a case can also be coped with by setting an upper limit to the switching frequency by any means. However, the design of an overcurrent protection circuit for the DC-DC converter needs to anticipate a frequency margin to some extent. That is, the overcurrent protection circuit requires an overcurrent limitation function that can reliably cope with an overcurrent state even at frequencies higher than a frequency that is used in an ordinary operation.